Proteins in acidic milk beverages such as “liquid yogurt”, lactic acid bacteria beverages, fruit milk and the like are highly unstable at near their isoelectric point of pH 3.8-5.3, such that the milk proteins coagulate, and with passage of time the milk proteins precipitate resulting in separation of the whey. Upon sterilization heating, this coagulation becomes severe to a point where the product value is totally lost.
In the past, it has been common to use pectin at 0.2-0.4 wt % in such acidic milk beverages such as liquid yogurt and lactic acid bacteria beverages in order to achieve stable dispersion of the proteins.
Pectin is an acidic polysaccharide typically present in high quantities in citrus fruits, apples, quince and the like, and galacturonic acid in the molecules can form crosslinking between proteins, resulting in stable dispersion of the proteins in the acidic milk beverages for given periods of time.
However, while addition of pectin can improve the stability of acidic milk beverages, it also increases their viscosity at the same time, resulting in a pasty feel, impairing the swallowability, or in extreme cases causing gelling, such that the manageability and product value are reduced.
There have been reported attempts to add low molecular pectin, subjected to enzymolysis or the like, to acidic milk beverages in order to reduce the viscosity of the acidic milk beverages (Carbohydrate Polymers, 6, P.361-378(1986), Japanese Unexamined Patent Publication No. 7-264977), but in all such cases it has been indicated that the stability is lowered with reduced molecular weight and that the use of low-molecularized pectin alone cannot stabilize acidic milk beverages.
Other inventions relating to the use of low-molecularized pectin include an invention employing thermolytically low-molecularized pectin as an emulsifier (which exhibits surface activity in water-in-oil systems) or emulsion stabilizer (which stabilizes water-in-oil emulsions) (Japanese Unexamined Patent Publication No. 10-4894), and an invention employing low-molecularized pectin obtained by action of endo-polygalacturonase on pectin, as food fiber (Japanese Unexamined Patent Publication No. 5-252972); however, none of these inventions teach that low-molecularized pectin obtained by pectin degradation contributes to dispersion stability of milk proteins under acidic conditions.
Moreover, the maximum pH of acidic milk beverages that can be stabilized by conventional pectin is at highest the isoelectric point of the proteins used, or up to pH 4.5 in the case of milk proteins, and since there has been no technique for stabilization of acidic milk beverages without production of a pasty feel in a pH range which is higher than the isoelectric point, there has been a limit to the flavors of acidic milk beverages that can be used.